Voltage-controlled oscillators (VCOs) are typically used in applications that require the generation of high frequency signals, e.g. in communication or sensor systems such as radar. One of the aspects by which the quality of a VCO is judged is the phase noise of the generated oscillation signal, as the phase noise can significantly limit the performance of a VCO-based system. Indeed a low phase noise is usually a chief requirement for any VCO.
FIG. 1 schematically shows an example of a simple generic VCO circuit 10 of the Colpitts type. A detailed discussion of this specific form of VCO has been given by Hao Li and Hans-Martin Rein, “Millimeter-Wave VCOs With Wide Tuning Range And Low Phase Noise, Fully Integrated in a SiGe Bipolar Production Technology”, IEEE Journal of Solid State Circuits, Vol. 38, p. 184 (2003) [Ref. 1]. Therefore only the basic features thereof will be summarized, for the sake of completeness. The oscillator circuit 10 comprises an inductor 32 (referred to here as the first inductor), a varactor 18, and a negative resistance element 30 in the form of an NPN bipolar transistor. The first inductor 32 couples a virtual ground 34 to the base of the bipolar transistor 30. The transistor's 30 emitter is coupled to the varactor 18, which is reverse-biased by means of a potential applied at a tuning terminal 22. A node 16 between the transistor's 30 emitter is coupled to the ground 12 via a current source 14. The transistor's 30 collector is coupled via a matching circuit 26 to an output terminal 24 at which a load (not shown) may be coupled. A supply terminal 28 for applying a voltage with respect to the ground 12 is coupled to the collector of the transistor 30 via the matching circuit 26. The inductor's 32 inductance L and the capacitor's 36 capacitance C together essentially determine the resonance frequency of the circuit. By varying the potential at the terminal 22, the varactor's capacitance C and hence the frequency of the circuit can be varied. The varactor can be any variable capacitance diode known in the art. The whole circuit 10 may be symmetric under a reflection by a symmetry axis 20 to provide a differential output between the first output terminal 24 and a corresponding output terminal 24′ (not shown). Finally it is noted that the instantaneous electric potential at a selected point within the circuit 10, that is, the voltage between the selected point and ground potential 12, is generally oscillatory but not zero on average. In the following a potential averaged over one oscillation period shall be referred to as a “direct current (DC) potential”.
A drawback of the circuit 10 is that in order to reverse-bias the varactor 18, the tunable potential VTUNE applied at the varactor 18 by means of the terminal 22 needs to be larger than VE, the potential at the emitter of the transistor 30. In order to provide the desired large tuning range for the tuning potential, the maximal tuning potential thus needs to be larger than the supply voltage VC applied at the supply terminal 28, typically 5 V or 3.3 V. Therefore, providing a sufficiently large tuning voltage requires additional efforts, such as providing an additional supply voltage, which increases the costs for practical applications.
Attempts have already been made to overcome the need for an additional supply voltage in a VCO. Referring now to FIG. 2, there is shown a VCO circuit of the type discussed above, however incorporating additionally a bias branch 36, 38 shortened to the ground 12. The branch's other terminal 36 is coupled to the varactor's terminal that is not coupled to the terminal 22 at which the tuning potential VTUNE is applied. The AC-coupling capacitor 40 and DC-bias resistor RBIAS 38 allow for a voltage between the nodes 16 and 36, so that the DC potential at the node 36 may be lower than VE, the DC potential at the emitter of the transistor 30. The current flowing in the resistor 38 being very small, the tuning potential may be set from 0 to its maximal value VTUNE.MAX, which now coincides essentially with the supply voltage VC. Contrary to the circuit of FIG. 1, an additional voltage source is not needed.
However, this circuit has a number of shortcomings. Firstly, the resistor 38 introduces new noise to the oscillator core. For achieving a low phase noise the resistor's 38 resistance RBIAS needs to be optimized. If RBIAS is too small, the varactor's quality factor is degraded, resulting in a worse phase noise. On the other hand, if RBIAS is too large the noise contribution from the resistor itself is large. An optimized resistor 38 value RBIAS can be found by making a compromise between the quality factor of the varactor and the noise contribution of the resistor. Indeed computations and measurements show that the single-side band (SSB) phase noise is minimum for a unique positive value of RBIAS. Nevertheless, even when the resistance RBIAS is optimized, the phase noise of the VCO is worse compared to the circuit of the type shown in FIG. 1, which does not have a bias branch.